Turkish jets conducted more than 20 overnight air strikes in northern Syria, pummeling a group of US-backed Kurdish militias that have been fighting the so-called Islamic State there.

The strikes against Kurdish-led fighters in the Syrian Democratic Forces on Wednesday night, which targeted militia positions in three villages northeast of Aleppo, constituted a major escalation by Turkey, just as an effort to recapture the city of Mosul from IS militants was intensifying in neighboring Iraq, where Kurdish groups have carried the brunt of the fighting.

American support of the Kurdish fighters has infuriated Turkish leaders, who see the militias as a terrorist organization tied to the Kurdish militants who have staged attacks within southeastern Turkey over the past three decades.

"From now on we will not wait for problems to come knocking on our door," Turkish President Tayyip Erdoğan vowed in a speech on Wednesday. "We will not wait until the blade is against our bone and skin, we will not wait for terrorist organizations to come and attack us."

The Turkish military reported that its warplanes killed between 160 and 200 fighters – a death toll reported Thursday by Turkey's state-run news agency – while the Britain-based Syrian Observatory for Human Rights monitoring group reported a much lower toll of 11 dead and dozens of wounded.

Commander Mahmoud Barkhadan, a senior commander of the main Kurdish militia in Syria, the People's Protection Units, told the Associated Press that early reports suggest no more than 10 fighters had been killed and 20 wounded.

"We will not back down," Mr. Barkhadan told the AP in a phone interview from the area, accusing Turkey of shifting the focus away from Daesh, which is the Arabic acronym for the Islamic State. "We are fighting Daesh, why are they striking at us?"

The latest developments, which come ahead of a visit Friday to Ankara by US Defense Secretary Ash Carter, highlight how world powers are pursuing conflicting agendas on an increasingly complex battlefield – a fact that has strained ties between the NATO allies, as The Christian Science Monitor's Scott Peterson reported in August:

But friction with the United States is just one issue for Turkey in a complex, multilayered battlefield. Other risks, analysts say, include inciting more regional Kurdish resentment that could worsen Turkey’s fight against Kurdistan Workers Party (PKK) militants inside Turkey, and military overreach inside Syria that yields more casualties than Turks are ready to bear.

Those risks are separate from Turkey’s broader strategic desire to see the end of Syrian President Bashar al-Assad’s rule – an aim that today seems all but forgotten as Turkey focuses instead on Kurdish autonomy and IS as bigger threats.

“There is an almost inevitable escalatory track in terms of [Turkey’s] troop commitment and the military’s presence in Syria,” says Julien Barnes-Dacey, a senior policy fellow in Brussels with the European Council on Foreign Relations.

“So much will depend on what the Turks achieve in the next weeks or months, because at the moment it seems that there is no exit strategy,” says Mr. Barnes-Dacey.

While the United States considers the Kurdish militia group to be the best force to take on IS in Syria, Turkish leaders have grown wary of the militia's success in securing large swaths of land along the Syrian-Turkish border.

When Turkey, a main backer of the insurgency against Syrian President Bashar al-Assad, entered the Syrian conflict in August, it used armor and air power to help rebel groups take control of territory near the border.But it also sought to prevent the Kurdish groups from gaining more ground.

Turkey fears the Kurds will try to unite three separate areas to create a Kurdish enclave in northern Syria, which could spur a separatist movement among Kurds on Turkish soil.

Material from Reuters and The Associated Press was included in this report.